1. Field of the Invention
The present invention relates to a projection color display apparatus which uses a plurality of light valves for forming an image.
2. Description of the Related Art
FIG. 1 schematically shows the structure of a conventional projection color display apparatus, and FIG. 2 is an enlarged partially sectional view of the structure of a part of a light source portion 1 of the apparatus. The light source portion 1 is composed of a white light source 2 such as a metal-halide lamp and a parabolic reflecting mirror 3 with a concave surface for converting the light flux emitted from the light source 2 into an approximately parallel light flux. The light source 2 has a discharge electrode 4 which is connected to a power source 5. The discharge electrode 4 is supported by a transparent outer wall 6 made of, for example, silica glass, and light is emitted from a light-emitting portion 7. The light source 2 is disposed such that the light-emitting portion 7 is situated in the vicinity of the focal point of the parabolic reflecting mirror 3.
From the light source portion 1, a light flux 8 having a circular section is emitted. A dichroic mirror 9 which has a wavelength selectivity for reflecting a red light flux 13R in the approximately orthogonal direction and transmitting a mixed light flux 10 of blue and green light fluxes is provided in the path for the light flux 8. In the path for the red light flux 13R, a reflecting mirror 11 is provided which reflects the red light flux 13R approximately orthogonally to the direction of the progress as a light flux 14R. In the path for the light flux 10, a dichroic mirror 12 is provided which has a wavelength selectivity for reflecting a blue light flux 14B in the approximately orthogonal direction and transmitting a green light flux 14G. In the paths for the light fluxes 14R, 14B and 14G, transmission light valves 15R, 15B and 15G are respectively provided so as to two-dimensionally modulate the light intensity in the planes which are orthogonal to the optical paths. Each of these light valves is constituted by an image display panel which utilizes, for example, an electrooptic effect of a liquid crystal for modulating a transmittance. The light valves 15R, 15B and 15G are driven in accordance with TV signals or the like, display TV images or the like and spatially modulate the intensity (sectional intensity) of the transmitted light which are emitted from the light valves. The size of each of the light valves is for example, 1 to 5 inches.
In this way, light fluxes 14R, 14B and 14G with the respective sectional intensities two-dimensionally modulated are emitted from the light valves 15R, 15B and 15G as light fluxes 16R, 16B and 16G, respectively. In the path for the light flux 16G, a reflecting mirror 17 is provided which reflects the light flux 16G approximately orthogonally to the direction of progress as a light flux 18G. A dichroic mirror 19 transmits the light flux 16R and reflects the light flux 16B approximately orthogonally to the direction of progress, thereby forming a light flux 20 which is a mixture of the light flux 16R and the light flux 16B. A dichroic mirror 21 transmits the light flux 20 and reflects the light flux 18G approximately orthogonally to the direction of progress, thereby forming a light flux 22 which is a mixture of the light flux 20 and the light flux 18G. The light flux 22 is a mixture of red, blue and green light fluxes with the respective intensities modulated by the light valves 15. A projection lens 23 projects the light flux 22 onto a screen 24 so that the real images of the images on the light valves 15R, 15B and 15G are projected on the screen 24 as an enlarged image. The size of the screen used is, for example, about 20 to 200 inches. The light valves 15R, 15B and 15G are disposed at an equal distance (optical path length) from the projection lens 23. The dichroic mirror utilizes the optical interference by a thin film and only reflects the light having a specific wavelength while transmitting the light of the other wavelengths.
The operation of the conventional apparatus will now be explained.
The light flux emitted from the white light source 2 is reflected by the parabolic reflecting mirror 3 with a concave surface as an approximately parallel light flux and converted into red, blue and green approximately parallel light fluxes 14R, 14B and 14G by the dichroic mirrors 9 and 12. The intensities of the red, blue and green light fluxes 14R, 14B and 14G are two-dimensionally modulated by the light valves 15R, 15B and 15G, respectively. The images on the light valves are enlarged and formed on the screen 24 by the projection lens 23, thereby forming an enlarged TV image or the like.
The light source portion of a conventional projection color display apparatus uses a parabolic reflecting mirror having a concave surface so as to converge the light flux emitted from the light source with efficiency and to obtain parallel rays. In order to enhance the utilization efficiency of the light flux emitted from the light source in the converging system, the light source is preferably as close to a point source as possible. If a metal-halide lamp is used as a light source, such a desirable condition is obtained by shortening the discharge gap length. However, when the discharge gap length is shortened, the making power per unit volume must be increased so as to obtain the same emission power, thereby shortening the life of the metal halide lamp. A light source which is as close to a point source as possible and which has a sufficient life necessary for the apparatus has been conventionally designed and developed.
For this reason, a light source with the light-emitting portion having a certain length (e.g., 5 mm) is conventionally used as the light source. The light emitting source of a conventional projection color display apparatus, however, is disadvantageous in that since it uses such a light source, it is impossible to obtain completely parallel rays from the parabolic reflecting mirror having a concave surface. Therefore, the light which is deviated from the parallel rays is diffused on the way from the light source to the light valves, thereby making it impossible to obtain sufficient luminance on the screen.
As described above, the utilization efficiency of light fluxes is incompatible with the life of a conventional projection display apparatus, and it is impossible to realize an ideal point source.